Rolling element bearings are precision made components the stiffness of which is predictable in more than one direction such that the movement of the inner ring of the bearing with respect to the outer ring is predictable as a function of load, temperature and light thermal parameters.
Methods employing non-contact sensors such as magnetic sensors are known in the prior art, e.g. disclosed in the document JP 2008-215977 A.
In this technology, a sensor system optimized for determining a wheel speed for ABS (Anti-lock Braking System) or slip control employing the frequency of the sensor signal is used to additionally determine the load acting on the bearing. For this purpose, the amplitude of the signal obtained by the magnetic sensor is read out. The amplitude of the magnetic field acting between a magnetic sensor and an angle target ring depends on the axial distance between these elements and is used to determine the relative axial position of the rings.
Commercially available hub units with sensor bearings are optimized for wheel speed detection and the period of the magnetically active pattern on or in the target ring about 8 mm or more. This period will also be referred to as the “wavelength” of the target ring in the following. For typical diameters of 270 mm or more, this pitch leads to a sufficient angular resolution while ensuring a sufficiently high signal-to-noise ratio and neatly detectable pulses. The relatively large wavelength ensures a good signal to noise ratio within the elastic movement of the bearing under any load, but does not allow an accurate sensing of the load as it varies relatively little with the displacement between the magnetic ring and the magnetic sensor. A shorter wavelength would allow an improved change with distance.
When using standard target wheels for load detection as disclosed e.g. in JP 2008-215977 A, the characteristic function describing the dependency of the signal amplitude on the distance has a fairly shallow slope such that the resolution in the distance detection is low. The reason for this is obvious: since standard sensor wheels are optimized for wheel speed detection, the signal should be unaffected by variations in the distance as far as possible.
On the other hand, the application PCT/EP2010/00345 discloses a method employing strain gauges attached to the inner ring or to the outer ring of the bearing in order to measure elastic deformations of the bearing. The local deformation is a measure of the load variation with respect to a baseline. However, the baseline depends on temperature and thermal gradients induced by friction heat such that it is normally impossible to derive absolute loads from absolute strains.
As a consequence, both methods are unsatisfactory. While the displacement sensor method is unable to account of effects of local strains or thermal expansions while the conventional method using strain gages has problems in determining absolute values for the load.